1. Field of the Invention
The present invention relates to a titanium alloy product having a high strength and an excellent cold rolling property.
2. Description of the Related Art
Because titanium alloys have high specific strength and are excellent in corrosion resistance, they are used in a wide range of fields as members of aerospace instrument, members of chemical plant, and automotive members. An example of a typical titanium alloy includes Ti-6Al-4V alloy. This Ti-6Al-4V alloy is excellent in strength properties, as the 0.2% proof stress of 828 MPa or more is standardized in ASTM Gr. 5;however, is poor in a cold rolling property because a large amount of Al is comprised as an additive element. Accordingly, it is difficult to manufacture a thin plate of the alloy by coil rolling, and is processed into a thin plate by a process generally called pack rolling. In this pack rolling, titanium plates obtained by hot rolling are piled up in layers to be wrapped up with a mild steel cover, and then hot rolled while the temperature thereof is being kept not to be lower than a predetermined one, thereby allowing titanium plates to be manufactured. In this process, there are problems that the work is extremely complicated in comparison with cold rolling and the process needs a lot of expenses. Further, there are many restrictions in terms of processing, because the temperature range suitable for the hot rolling is limited.
On the other hand, an example of a general-purpose titanium alloy that can be coil rolled includes, for example, Ti-3Al-2.5V alloy (ASTM Gr. 9). However, the 0.2% proof stress of this alloy is approximately 500 MPa, which is considerably smaller than that of the aforementioned Ti-6Al-4V alloy. In addition, Japanese Patent Publication No. Hei 2(1990)-57136 discloses a heat-resistant Ti alloy plate excellent in cold workability. This alloy plate has been developed for the first purpose of improving cold workability, and the additive content of each of an α-stabilizing element and a β-stabilizing element is low. Accordingly, an increase in strength by solute strengthening is small, and hence it is difficult to use this alloy plate in an application in which a high strength is required.
On the other hand, as a titanium alloy that has a strength similar to that of the Ti-6Al-4V alloy and that can be coil rolled, KSTi-9 (Ti-4.5Al-2Mo-1.6V-0.5Fe-0.3Si-0.05C, ASTM Gr. 35, Japanese Patent No. 3297027) has been developed, and cold rolled coils thereof are actually manufactured on a mass-production scale. Similarly to the Ti-6Al-4V alloy, Mo and V are used as β-stabilizing elements in the KSTi-9. In addition, an example of a high strength Ti alloy includes Ti-4Al-2.5V-1.5Fe-0.250 (ATI 425 (U.S. registered trademark)). In this Ti alloy, V is used as a major β-stabilizing element (β-strengthening element).
Further, Japanese Unexamined Patent Publication No. Hei 1(1989)-111835 discloses an alloy that has been developed for the purpose of improving cold workability. In the Ti alloy disclosed therein, the additive content of a β-stabilizing element is high to obtain high workability thanks to a residual β-phase.